System and method for data distribution in vhf/uhf bands

ABSTRACT

Whitespace devices can use unused television frequencies for transmission and reception of WiFi OFDM signals. Three contiguous bands, such as former channels 2, 3, and 4, may be bonded together to define a whitespace band. In order to fit a WiFi OFDM signal into this whitespace band, a whitespace device compresses the bandwidth of each WiFi OFDM signal using a specific spectrum mask. Very low transmission power is needed for the modified WiFi OFDM signals, eliminating the need for high power amplifiers and most of the WiFi OFDM designs such as PHY and MAC can be reused with minor modifications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/245,209, filed Oct. 3, 2008, titled “System and Method for DataDistribution in VHF/UHF Bands,” which is hereby incorporated byreference.

BACKGROUND

This invention relates generally to distribution of data signals in ahome environment, and more particularly to a system and method for datadistribution in the VHF/UHF band.

WiFi (Wireless Fidelity) is the trade name for the global set of 802.11standards drafted for wireless Local Area Networks (LAN); any standardWi-Fi device will work anywhere in the world. WiFi is one of the mostpopular wireless technologies; it is widely available in publichotspots, homes, and campuses worldwide, being supported by nearly everymodern personal computer, laptop, most advanced game consoles, printers,and many other consumer devices. Routers which incorporate a DigitalSubscriber Line (DSL) modem or a cable modem and a Wi-Fi access point,often set up in homes and other premises, provide Internet access andinternetworking to all devices connected (wirelessly or by cable) tothem.

Wi-Fi uses both single carrier direct-sequence spread spectrum radiotechnology (812.11b) and multi-carrier Orthogonal Frequency Divisionmultiplexing (OFDM) radio technology (e.g. 802.11a, g, j, n). TheInstitute for Electronic and Electrical Engineers (IEEE) has establisheda set of standards for Wireless Local Area Network (WLAN) computercommunication, collectively known as the IEEE 802.11 standard that areapplicable to Wi-Fi signals.

The 802.11a standard uses OFDM radio technology in the 5 GHz U-NII band,which offers 8 non-overlapping channels and provides data rates of up to54 Mbps. Another standard that uses OFDM is 802.11g, which attempts tocombine the best features of the 802.11a and 802.11b standards. It usesenables data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps, and the 2.4GHz frequency for greater range. The 802.11j standard is an amendmentdesigned especially for Japanese markets. It allows WLAN operation inthe 4.9 to 5 GHz band to conform to Japanese rules for radio operationfor indoor, outdoor, and mobile applications. Finally, the 802.11nstandard is a proposed amendment which improves upon the previous 802.11standards by adding multiple-input multiple-output (MIMO) and many othernewer features. Though there are already many products on the marketbased on the latest draft of this proposal, the 802.11n standard willnot be finalized until December 2009.

In the U.S., 802.11a and 802.11g devices may be operated without alicense. The 802.11a standard uses 20 MHz channels and operates in threeunlicensed bands, known as the Unlicensed National InformationInfrastructure (U-NII) bands; four 20 MHz channels are specified in eachof these bands. The lower U-NII band, extending from 5.15 to 5.25 GHz,accommodates four channels with a 40 mW power limit; the middle U-NIIband, extending from 5.25 to 5.35 GHz accommodates four channels with a200 mW power limit; and the upper U-NII band, extending from 5.725 to5.825 GHz, accommodates four 20 MHz channels with an 800 mW power limit.

An 802.11a signal uses OFDM modulation with 52 subcarriers, whichinclude 48 data subcarriers and four pilot subcarriers; the subcarrierscan be modulated using BPSK, QPSK, 16 QAM, or 64 QAM. The total symbolduration is 4 μs, and includes a useful symbol duration of 3.2 μs and aguard interval of 0.8 μs, with a peak data rate of 54 Mbps. Subcarriersare spaced apart by 312.5 kHz so that the signal actually occupies abandwidth of 16.25 MHz in theory.

In the United States, there are roughly 210 television (TV) broadcastregions and 1700 TV broadcasting stations. Currently, each TV station isassigned around eight radio frequency (RF) channels for NTSC broadcast,each channel occupying 6 MHz in the VHF/UHF spectrum. The FederalCommunications Commission (FCC) has mandated that all full-powertelevision broadcasts will use the Advanced Television Systems Committee(ATSC) standards for digital TV by no later than Feb. 17, 2009. All NTSCtelevision transmissions will be terminated by that date. Following theNTSC TV switch-off, the FCC will allocate channels 2 through 51 todigital TV; channels 52 through 69 that occupy the lower half of the 700MHz band have been already reallocated through auction to variousadvanced commercial wireless services for consumers.

The ATSC standard mandates a bandwidth of 6 MHz for each TV channel, useof Trellis Eight-Vestigial Side Band (8-VSB) modulation, andReed-Solomon encoding. The TV receiver has some basic requirements toproperty decode the ATSC signal and provide good quality TV pictures.These requirements include that the TV Signal to Noise Ratio (SNR) is noless than 15.2 dB, a thermal noise floor of −106.2 dBm (dBm is theabbreviation for the power ratio measurement units), and a sensitivityof between −81 and −84 dBm etc.

As each TV station operating in a certain geographic region/area usesonly a limited number of channels from the TV band, some digitalchannels remain unused in the respective area: this locally availablespectrum is called “whitespace.”

It is expected that the FCC will allow the whitespace bands to be usedonly by devices that do not interfere with existing TV broadcast,wireless microphones, or Global Positioning System (GPS) systemsdeployed in that area. Consequently, the signals radiated by anywhitespace devices/equipment operating in the ATSC spectrum must followthe FCC regulations so that the quality of the primary TV service willnot be degraded by the signals using the nearby whitespace. Thus, thenew whitespace devices should be designed so as to not affect the TVtuner sensitivity (−81˜−84 dB) and the TV receiver performance atSNR=15.2 dB.

A known solution for distributing multimedia content within a home iswireless high definition TV (HDTV). However, wireless HDTV requires avery high data rate (greater than 1 Gbps) and the 60 GHz band is notsuitable for transmission over distances greater than 10 m. In addition,the quality of such a wireless link is not satisfactory and the cost ishigh.

Another known solution for distributing data and video within a home isWiFi. However, the WiFi band has uncontrollable interferences and thequality cannot be guaranteed.

Thus, there is a need to provide an inexpensive and efficient way tobroadcast multimedia content within a confined environment, usingwireless solutions. There is also a need to recycle the spectrum that isnot used in a certain geographical area.

SUMMARY

Some simplifications and omissions may be made in the following summary,which is intended to highlight and introduce some aspects of the variousexemplary embodiments, but not to limit the scope of the invention.Detailed descriptions of a preferred exemplary embodiment adequate toallow those of ordinary skill in the art to make and use the inventiveconcepts are provided by the entire disclosure.

It is an object of the invention to provide a method and system forwireless distribution of data and/or video within a home using OFDMtechnology, and in particular using WiFi OFDM signals. While thespecification describes WiFi OFDM variants of the invention; it is to beunderstood that the invention applies to other technologies and is notrestricted to WiFi OFDM signals.

Another object of the invention to provide a method and system forconfining (retrofitting) WiFi OFDM signals into the whitespace that willbecome available once digital TV signal is phased in. As discussed inthe Background section of this specification, the bandwidth used by theWiFi OFDM signals in the 5 GHz band is 20 MHz, and is therefore,slightly greater than the bandwidth of three consecutive TV channels inNorth America, which is 3×6=18 MHz. Also, a standard WiFi OFDM signalcannot fit within the whitespace band of three consecutive TV channelsdirectly, due to high shoulders of its signal spectrum, which willseverely interfere with the adjacent TV channels. The solution disclosedhere confines WiFi OFDM signals into the whitespace, both in terms ofbandwidth and emitted power, without causing interference with theexisting TV broadcast.

It is to be noted that the invention described herein is equallyapplicable to whitespace of various widths. The particular example ofretrofitting a WiFi OFDM signal within an 18 MHz bands is a practicalsolution for North America, that result in minimal changes to theexisting WiFi device. However, the invention is not restricted to awhitespace of 18 MHz; applying the technique described here, narrowerwhitespace bands may be used. As well, as other countries that have adifferent TV channel width, whitespace freed by two TV channels may beused according to the invention. For example, a TV channel in Japanoccupies 8 MHz, so that there is no need to use the whitespace freed bythree TV channels; two could be enough. As well in the width of a TVchannel in European countries is 7 MHz; in this case the WiFi OFDMsignal can be used with the more relaxed embodiment of this invention,or less than three TV channels may be used by modifying the spectrummask according to the invention in an appropriate way.

Still another object of the invention is to provide wirelessdistribution of data and/or video with minimal changes to the hardwareof the existing WiFi devices.

In various exemplary embodiments, a method of transmitting user dataover a local area network (LAN) within a VHF/UHF band may compriseidentifying in the VHF/UHF band a whitespace band B_(WS) available in anarea of operation of the LAN; generating a baseband WiFi OFDM signalfrom user data; reconfiguring the baseband WiFi OFDM signal into amodified WiFi OFDM signal using a transmit spectrum mask adapted toconfine the bandwidth of the modified WiFi OFDM signal into thewhitespace band B_(WS), and to attenuate the modified WiFi OFDM signalat the edges of the whitespace band B_(WS) for maintaining a performanceof any neighboring TV channel unchanged; and transmitting the modifiedWiFi OFDM signal over the whitespace band.

Advantageously, the invention provides a solution for reusing thewhitespace available in a respective area, at low cost and with a betterperformance than the current solutions. These advantages result from useof the lower part of the spectrum (VHF/UHF rather than 5 GHz), whichresults for example in a simplified design of the RF part of theexisting devices. This is because at lower frequencies, the distances atwhich signals may be transmitted are greater that in the higherfrequency bands; a direct result is that the transmitter design may useonly a preamplifier rather than a power amplifier as in the currentdesigns, resulting in a cost decrease.

The foregoing objects and advantages of the invention are illustrativeof those that can be achieved by the various exemplary embodiments andare not intended to be exhaustive or limiting of the possible advantageswhich can be realized. Thus, these and other objects and advantages ofthe various exemplary embodiments will be apparent from the descriptionherein or can be learned from practicing the various exemplaryembodiments, both as embodied herein or as modified in view of anyvariation that may be apparent to those skilled in the art. Accordingly,the present invention resides in the novel methods, arrangements,combinations, and improvements herein shown and described in variousexemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is next described with reference to the followingdrawings, where like reference numerals designate corresponding partsthroughout the several views.

FIG. 1A shows the eight WiFi carriers in the lower and middle U-NIIbands;

FIG. 1B shows the four WiFi carriers in the upper U-NII band;

FIG. 2 depicts the transmit spectrum for a WiFi OFDM signal;

FIG. 3 illustrates the U.S. ATSC broadcast band;

FIG. 4 is a flowchart of the method according to an embodiment of theinvention;

FIG. 5 depicts an emission mask for a WiFi OFDM signal according to anembodiment of the present invention;

FIG. 6 depicts an alternative emission mask for an exemplary WiFi OFDMsignal according to another embodiment of the present invention;

FIG. 7A shows an exemplary transmitter according to an embodiment of thepresent invention;

FIG. 7B shows the RF units of a conventional WiFi transmitter that arereplaced in the transmitter shown in FIG. 7A;

FIG. 8A shows an exemplary receiver according to an embodiment of thepresent invention; and

FIG. 8B shows the RF units of a conventional WiFi receiver that arereplaced in the receiver shown in FIG. 8A.

DETAILED DESCRIPTION

Referring now to the drawings, in which like numerals refer to likecomponents or steps, there are disclosed broad aspects of variousexemplary embodiments. It is again noted that WiFi and North AmericaATSC standard are used by way of example. Other OFDM signals may beretrofitted in the whitespace freed by transition to digital TV in otherparts of the world. Also, use of whitespace provided by threeconsecutive TV channels is described here as the preferred embodiment ofthe invention: whitespace of different width may also be used fortransmitting OFDM signals in the VI-IF/UHF spectrum.

FIGS. 1A and 1B show the 802.11a carriers in the 5 GHz band, where FIG.1A shows eight WiFi carriers C1-C8 in the lower and middle U-NII bands,and FIG. 2 shows the four WiFi carriers C9-C12 in the upper U-NII band.Each central frequency is spaced 20 MHz relative to neighboringcarriers. In FIG. 1A, for a lower band edge of 5.15 GHz and an upperband edge of 5.35 GHz, the total bandwidth is 200 MHz. The first centralfrequency C1 is 30 MHz above lower edge of the lower U-NII band and theeighth central frequency C8, is 30 MHz below the upper edge of themiddle U-NII band. In the upper U-NII band shown in FIG. 1B, the totalbandwidth is 100 MHz, extending between a lower band edge of 5.725 GHzand an upper band edge of 5.825 GHz. The first carrier C9 is 20 MHzabove the lower edge of the upper U-NII band and the fourth carrier C12is 20 MHz below the upper edge of the band.

Besides the central frequency of each channel, the 802.11 standard alsospecifies a spectral mask defining the permitted distribution of poweracross each channel. FIG. 2 depicts the transmit spectrum mask 20according to the 802.11a standard and the power spectrum 25 of a typicalWiFi OFDM signal. As shown in FIG. 2, the mask has a maximum plateau 21at 9 MHz around the central frequency F_(C). Then, the signal isattenuated by about 20 dBr (“dBr” stands for “relative”) from its peakenergy in the range of 9-11 MHz from the central frequency F_(C), shownat 22, 22′, so that in practice the channels are effectively 22 MHzwide. A smaller rate of power decay creates a skirt 23, 23′ over therange 11-20 MHz away from F_(C), where the power level only drops from−20 dBr to −28 dBr. The mask then declines below −40 dBr, shown at 24,24′ at frequencies more than 30 MHz away from f. As seen in FIG. 2, thewide skirt 24, 24′ of a standard WiFi OFDM signal extends well outsideof the 20 MHz range. It is however assumed that the signal on anychannel is sufficiently attenuated outside the 20 MHz bandwidth tominimally interfere with a transmitter on any other channel.

FIG. 3 illustrates the U.S. digital television broadcast band after Feb.17, 2009. The ATSC television signals will be broadcast in the VHF (veryhigh frequency) band and/or the lower part of the UHF (ultra highfrequency) band. As seen in FIG. 3, the digital TV channels are groupedinto five bands denoted with T1-T5. The band T1 occupied by channels 2-4has 18 MHz, extending from 54 MHz to 72 MHz. The band T2 occupied bychannels 5-6 has 12 MHz between 76 MHz to 88 MHz, the band T3 occupiedby channels 7-13 has 42 MHz, between 174 MHz and 216 MHz. Further, theband T4 carrying channels 14-36 occupies 138 MHz, extending from 470 MHzto 608 MHz and the band T5 occupied by channels 38-51 has 84 MHz, from614 MHz to 698 MHz. Thus, this group of 49 channels covers a totalspectrum of 294 MHz (18+12+42+138+84).

Since channels 2, 3, and 4 will be reserved for some specificapplications, after this reservation, the commercial ATSC TV channelswill encompass 274 MHz, ranging from 76 MHz to 698 MHz, as shown in grayon the lower part of FIG. 3.

One embodiment of the present invention includes analyzing bandwidthallocation in the VHF/UHF band, detecting a frequency band denotedgenerally with B_(WS) that is unused, and transmitting data and videoover a WiFi OFDM signal in this unused bandwidth. In the case that theavailable whitespace is 18 MHz (e.g. the bandwidth not used by threeconsecutive RF channels based on the North America TV standards), oneembodiment of the invention reconfigures the WiFi OFDM signal in orderto retrofit a 20 MHz WiFi signal into the 18 MHz band of these threeconsecutive TV channels.

In addition, band T1 occupied by channels 2, 3, and 4 may also becameavailable as whitespace for use by other applications. T1 hastraditionally been set aside for set top boxes or Video CassetteRecorders (VCRs), Digital Versatile Discs (DVDs), etc. However, T1 maystay free most of the time, once non-radio frequency (RF) means of TVsignal transport, such as the High Definition Multimedia Interface(HDMI), become prevalent.

FIG. 4 shows a flowchart depicting the steps of the method according toone embodiment of the invention. In step 100, the whitespace is detectedusing, for example, a wavelet analyzer as described in U.S. patentapplication Ser. No. 12/078,979, titled “A System and Method forUtilizing Spectral Resources in Wireless Communications,” filed Apr. 9,2008, which is incorporated herein by reference. The wavelet analyzer isoperable to monitor the wireless signals present within the frequencyand time domains of a communication spectrum (here, the VHF/UHFspectrum) with a view to automatically and continuously identifybandwidth that is not used currently (whitespace) in the area ofinterest. It is to be noted that other means for identifying idlebandwidth suitable for the transmission of WiFi OFDM signals may also beused, without departing from the scope of this invention.

In step 110, it is established if a whitespace bandwidth correspondingto three consecutive TV channels is available. As shown by branch “No”of the decision block 110, the search for identifying whitespaceextending over three consecutive TV channels continues until successful;it is to be noted that since the number of TV channels broadcast in eachgeographical area is limited (currently there are 8 TV channels perstation), the likelihood to find such whitespace is quite high.

As one illustrative example, assume that three free consecutive channelsare identified as shown by branch “Yes” of decision block 110; forexample, these are channels C8, C9 and C10 from band T3 (see FIG. 3). Inthis case, step 130 is performed next since these channels are not TVchannels C2-C4, as established at decision block 120. These free TVchannels occupy 18 MHz, and as discussed above, a WiFi OFDM signalnormally requires a 20 MHz bandwidth and has a wide skirt that extendswell beyond this range. According to this embodiment of the invention,standard signals are modified so as to retrofit them into the 18 MHzband, as shown by step 130. The modified WiFi OFDM signal is alsoformatted so as to be consistent with all FCC requirements regardinginterference with neighboring TV channels.

Next, the modified WiFi OFDM signal is adapted for transmission in thewhitespace identified in step 100. This means that the baseband WiFiOFDM signal is modulated on subcarriers selected in the whitespace, asshown in step 140, and then transmitted over the whitespace band in step150. Details on how the WiFi OFDM signal is modified and adapted fortransmission in this whitespace band will be described in further detailin connection with FIG. 5, which provides a novel emission mask forretrofitting a 20 MHz standard WiFi OFDM signal into an 18 MHz band.

If the free channels identified in step 100 are TV channels C2-C4, asshown by branch “Yes” of decision block 120, step 140 and 150 areperformed, whereby the WiFi OFDM signal is adapted for transmission inthe whitespace otherwise occupied by C2-C4. Details on how the WiFi OFDMsignal is adapted for transmission in this whitespace band will bedescribed in further detail in connection with FIG. 6.

FIG. 5 depicts a novel emission mask 500 designed for a WiFi OFDM signal550 according to an embodiment of the present invention. FIG. 5 showsthe sub-channels of the WiFi OFDM signal centered about the channelfrequency denoted with F_(C). As discussed previously, the WiFi OFDMsignal uses 52 subcarriers (and 12 null subcarriers). In thisembodiment, three consecutive idle TV channels are selected fortransmission of the WiFi OFDM signal; these channels could be, as in theabove example, channels C8, C9, and C10 of band T3 from FIG. 3. Theselection is made based on the assumption that channels C8-C10 are notused locally for transmission of ATSC TV signals. It is to be noted thatthese channels are in the middle of the T3 band, and as such,neighboring ATSC TV channels C7 and C11 may be active. Consequently, theemission mask for this case must take into account the presence ofadjacent channels C7 and C11, and be designed such that the WiFi OFDMsignal does not detrimentally affect the quality of the adjacent TVchannels.

As shown in FIG. 5, the emission mask 500 according to this embodimenthas a somewhat different format relative to standard WiFi mask 20 shownin FIG. 2. As in the case of the spectrum mask 20, the signal plateau510 for the maximum level extends 9 MHz on both sides of the centralfrequency F_(C). However, the attenuation slope of the power curve shownby the skirts 520, 520′ is very high; the power level drops dramaticallyin a space of only 500 kHz, declining to −36 dBr at 9.5 MHz away fromthe central frequency F_(C). Power level continues to declinethereafter, as shown by slopes 530, 530′ reaching −99 dBr at 15 MHz awayfrom F_(C).

The WiFi OFDM signal 550 of embodiment shown in FIG. 5 has an upperguard band 554 of 2.5 MHz, protecting the adjacent TV channel at thehigher end of the 18 MHz whitespace band, and a lower guard band 552 of2.5 MHz, protecting the adjacent TV channel at the lower end of the 18MHz whitespace band. These guard bands 552, 554 are obtained with aproper implementation of the filters 706 (see FIG. 7A) which guaranteesthat any interference with adjacent channels meets the FCC interferenceregulations for TV usage in the whitespace band. FIG. 5 also shows at540 an ideal signal spectrum; it is to be noted that in practice,filters 706 may be designed to shape the signal spectrum between mask500 and the ideal spectrum 550.

The WiFi OFDM signal 550 is modified to match mask 500. In order toprovide the upper and lower guard bands 554, 552, the spectrum actuallyoccupied by the modified WiFi OFDM signal 450 between subcarriers 1 and52 is only 13 MHz instead of the 16.25 MHz that would have been occupiedby a standard WiFi OFDM signal. This results in a subcarrier spacing of250 kHz (13 MHz/52 subcarriers), which is lower that the subcarrierspacing of the standard WiFi OFDM signals of 312.5 kHz.

In this example, the useful symbol duration is lengthened from 3.2 μs ofthe standard WiFi OFDM signal to 4 μs and the guard interval betweensubcarriers is proportionately increased from 0.8 us to 1.0 μs. The peakdata rate is lower than for standard WiFi OFDM signals, dropping to 43.2Mbps instead of the standard 54 Mbps, due to the increase in symbolduration from 4 μs to 5 μs. This may require the system timers to bereset. However, the decrease in peak data rate is not likely to impactthe overall system throughput much, since the modified WiFi OFDM signaluses a lower frequency band (VHF/UHF) and therefore can better cope withthe environmental channel statistics.

FIG. 6 depicts an emission mask 600 used for an exemplary WiFi OFDMsignal 600 according to another embodiment of the present invention,suited for use in the TV band denoted with T1 in FIG. 3. As band T1 isused by only three digital TV channels, C2-C4, the design requirementsfor the WiFi OFDM signal in this band are more relaxed; there are no TVchannels to interfere with to the right or left of this band. Emissionmask 600 according to this embodiment is similar to the mask 500 shownin FIG. 5, but is translated at a different central frequency F_(C).FIG. 6 also shows at 640 an ideal signal spectrum according to thisembodiment of the invention; it is to be noted that in practice, filters706 (see FIG. 7A) may be designed to shape the signal spectrum betweenmask 600 and the ideal signal spectrum 650.

FIG. 6 provides the specific value of the frequencies from the spectralmask, because the position of the channels 2-4 in the spectrum is known.Since the requirements in band T1 are more relaxed, in this embodimentthe parameters of the WiFi OFDM signal 650 differ from the parameters ofthe WiFi OFDM signal 550. Thus, the bandwidth of signal 650 is 16.25MHz, the same as in the case of the standard signal, but it ranges from54.875 MHz to 71.125 MHz. The subcarrier spacing in this embodiment is312.5 kHz (16.25 MHz/52 subcarriers), again the same as in the case ofthe standard signal. The symbol duration and guaranteed data rate arealso consistent with the 802.11a and 802.11g standards, at 4 μs (3.2 μsfor the useful symbol duration and 0.8 μs for the guard intervalduration) and 54 Mbps, respectively.

FIG. 7A depicts an exemplary OFDM transmitter 700 according to oneembodiment of the invention. As shown in FIG. 7A, OFDM transmitter 700comprises a plurality of baseband blocks which may be similar to theblocks used by a conventional WiFi transmitter, such as a FEC encoder701, an interleaver 702, a constellation mapping block 703, an OFDMsymbol construction block 704, and an inverse Fourier transform block705. The part of the transmitter 700 denoted with 750 is, however,different from that of the corresponding part of a conventionaltransmitter shown in FIG. 7B.

A first difference is the design of the baseband filters 706 from thefilters 711 shown in FIG. 7B. As a preliminary matter, filters 706 areillustrated as one distinct unit only to provide a clear explanation ofthe frequency characteristics. As known in the art, signal filtering andshaping may be a multistage process rather than a one stage process.Also, filter 706 is not necessarily connected after the DAC 707.Alternatively, the DAC 707 may itself include filters that contribute tosignal shaping.

Filters 706 shape the WiFi OFDM signal according to the masks 500 or600, shown in FIG. 5 or FIG. 6, respectively. The differences betweenthe transmit spectrum mask 20 used for conventional WiFi OFDM signalsand the transmit spectrum mask 500 or 600 used for the modified WiFiOFDM signal of the invention were discussed previously.

Another difference is that transmitter 700 uses a low power amplifier ora preamplifier 708 that amplifies the symbols before modulation in themixer (multiplier) 709. While conventional WiFi systems require a highpower amplifier 714, as shown in FIG. 7 a, the present invention may usea less costly preamplifier, as little power is needed to broadcast for ashort distance (within a house) in the VHF/UHF band. Using an ultralowpower design to cover a home environment, power may be no more than, forexample, 200 mV/m.

Another difference appears in the structure of the mixers 709 of thetransmitter 700, as opposed to the mixer 713 of a conventionaltransmitter. Transmitter 700 uses subcarriers in the VHF/UHF band, asdiscussed in connection with FIGS. 5 and 6, rather than in the 2.4 GHzor 5 GHz bands used for the standard WiFi OFDM signals. Therefore, mixer709 should heterodyne the baseband signals to the center of thewhitespace band in the VHF/UHF band. For the example where channels C8,C9 and C10 are bonded together to form the white-space band as in FIG.5, mixer 709 should be designed to mix a VHF frequency corresponding tothe central frequency of the band occupied by these channels. For thecase when the channels 2, 3, and 4 are bonded together to form thewhitespace band as in FIG. 6, the desired frequency range extends fromroughly 54 MHz to 72 MHz. Mixer 709 should be designed to mix a VHFfrequency of approximately 63 MHz, corresponding to the centralfrequency of the band T1 in this example.

Also, a VHF/UHF antenna 710 is used for transmitting the WiFi OFDMsignals over short distances by transmitter 700, rater than an antenna715 used by the conventional WiFi OFDM signals that are transmitted inthe 2.4 GHz or 5 GHz bands over longer distances.

In the example of FIG. 7A, the size of the Fast Fourier Transformremains unchanged at 64, as the number of subcarriers used by themodified WiFi OFDM signal is still 64, namely 48 data subcarriers, 4pilots and 12 null subcarriers. Among these, the twelve null subcarriers(e.g. 0, 27-37) may be used for guard bands. The four pilot subcarriersmay, for example, be subcarriers 7, 21, 43, and 57.

FIG. 8A shows an exemplary OFDM receiver 800 according to one embodimentof the present invention. As shown in FIG. 8A, receiver 800 comprises aplurality of baseband units that are similar to the units used byconventional WiFi receivers. The RF part of the receiver, i.e. theantenna 801 and the RF receiving unit 802 differ from the correspondingunits used by the conventional WiFi systems shown in FIG. 8B.

Thus, VHF antenna 801 is adapted to receive incoming signals in the VHFband that are broadcast over relatively short distances. The receivingunit 802 includes a Low Pass Filter (LPF) 811 that removes highfrequency noise and passes the signals in the VHF band. Ananalog-to-digital converter (ADC) 812 of receiving unit 802 converts thereceived analog signal into a sequence of bits, a synchronizer 813converts the sequence of decoding bits into a sequence of frames ofbits, each of the sequence of frames having M decoding bits. Incontrast, the receiving unit 820 for standard WiFi OFDM signals, shownin FIG. 8B uses a WiFi RF filter 821 suitable for the respective 2.4/5GHz bands. As well, ADC 822 and synchronizer 823 of receiving unit 820are designed for recovering the baseband signals from the standard WiFiband, and not the VHF band allocated to the digital TV channels.

The baseband units used by the receiver 800 operate to perform thereverse operation on the baseband signals provided by the receiving unit802. Thus, Fast Fourier Transforming (FFT) unit 803 decodes the bits inthe sequence of frames to generate a sequence of symbol frames, each ofthe frames having at least N time domain decoded symbols. Channelestimation and equalization unit 804 and demapper 805 process thesequence of decoding symbol frames to generate a sequence of frames of Ninterleaved sub-channel bits, and deinterleaver 806 processes each ofthe frames of N interleaved sub-channel bits to generate a stream of Nrecovered bits. The FEC decoder 807 performs error correction anddescrambler 808 recovers the bits of the original signals.

From the above description, it will be apparent that the inventiondisclosed herein provides a novel and advantageous system and method fordata distribution in VHF/UHF band. The foregoing discussion disclosesand describes merely exemplary methods and embodiments of the presentinvention. One skilled in the art will readily recognize from suchdiscussion that various changes, modifications, and variations may bemade therein without departing from the spirit and scope of theinvention. Accordingly, disclosure of the present invention is intendedto be illustrative, but not limiting, of the scope of the invention,which is set forth in the following claims.

1. A method of transmitting user data within a communication band, overa local area network (LAN), comprising: identifying in the communicationband a whitespace band available in an area of operation of the LAN;generating a baseband WiFi orthogonal frequency division multiplexing(OFDM) signal from user data; reconfiguring the baseband WiFi OFDMsignal into a modified WiFi OFDM signal using a transmit spectrum maskthat defines the bandwidth of the modified WiFi OFDM signal in thewhitespace band and attenuates the modified WiFi OFDM signal at theedges of the whitespace band for maintaining a performance of anyneighboring licensed primary service; and transmitting the modified WiFiOFDM signal over the whitespace band.
 2. The method of claim 1, furthercomprising determining whether the whitespace band is in a first portionof the communication band, wherein when the whitespace band is in thefirst portion of the communication band, an effective spectrum occupiedby the modified WiFi OFDM signal is 16.25 MHz, and when the whitespaceband is not in the first portion of the communication band, an effectivespectrum occupied by the modified WiFi OFDM signal is 13 MHz.
 3. Themethod of claim 2, wherein the first portion of the communication bandincludes digital TV channels 2-4.
 4. The method of claim 1, wherein aneffective spectrum occupied by the modified WiFi OFDM signal is 16.25MHz.
 5. The method of claim 4, wherein the transmit spectrum mask has aplateau of 18 MHz around a central frequency to enable a maximumamplitude for the modified WiFi OFDM signal and an attenuation of −99 dBover 6 MHz from edges of the plateau.
 6. The method of claim 5, whereinthe transmit spectrum mask includes sharp attenuations of −36 dB at theedges of the plateau over 500 kHz.
 7. The method of claim 4, wherein themodified WiFi OFDM signal has a symbol duration of 4 μs, including 3.2μs as a useful signal duration and 0.8 μs as a guard interval duration.8. The method of claim 7, wherein the modified WiFi OFDM signal has asubcarrier spacing 312.5 kHz.
 9. The method of claim 4, wherein themodified WiFi OFDM signal includes a data rate of 54 Mbps.
 10. Themethod of claim 1, wherein an effective spectrum occupied by themodified WiFi OFDM signal is 13 MHz.
 11. The method of claim 10, whereinthe transmit spectrum mask has a plateau of 18 MHz around a centralfrequency to enable a maximum amplitude for the modified WiFi OFDMsignal and an attenuation of −99 dB over 6 MHz from edges of theplateau.
 12. The method of claim 11, wherein the transmit spectrum maskincludes sharp attenuations of −36 dB at the edges of the plateau over500 kHz.
 13. The method of claim 10, wherein the modified WiFi OFDMsignal has a symbol duration of 5 μs, including 4 μs as a useful signalduration and 1 μs as a guard interval duration.
 14. The method of claim13, wherein the modified WiFi OFDM signal has a subcarrier spacing of250 kHz.
 15. The method of claim 10, wherein the modified WiFi OFDMsignal includes a data rate of 43.2 Mbps.
 16. The method of claim 1,wherein the modified WiFi OFDM signal is modulated over 48 data carriersand uses 4 pilot carriers in the whitespace band.
 17. The method ofclaim 1, wherein the whitespace band extends over a band around acentral frequency in a portion of the spectrum allocated to digital TVbetween 54 MHz and 698 MHz.
 18. A transmitter that broadcasts user dataover a local area network (LAN) within a whitespace band in acommunication band, where the whitespace band is not currently used inan area of operation of the LAN for transmission of licensed primaryservices, comprising: a baseband generator configured to generate abaseband WiFi orthogonal frequency division multiplexing (OFDM) signalfrom the user data; a frequency converter configured to reconfigure thebaseband WiFi OFDM signal into a modified WiFi OFDM signal using atransmit spectrum mask that defines the bandwidth of the modified WiFiOFDM signal in the whitespace band and to attenuate the modified WiFiOFDM signal at the edges of the whitespace band for maintaining aperformance of any neighboring licensed primary service; and atransmitter configured to transmit the modified WiFi OFDM signal overthe whitespace band.
 19. The transmitter of claim 18, wherein when thewhitespace band is in a first portion of the communication band, aneffective spectrum occupied by the modified WiFi OFDM signal is 16.25MHz, and when the whitespace band is not in the first portion of thecommunication band, an effective spectrum occupied by the modified WiFiOFDM signal is 13 MHz.
 20. The transmitter of claim 19, wherein thefirst portion of the communication band includes digital TV channels2-4.
 21. The transmitter of claim 18, wherein an effective spectrumoccupied by the modified WiFi OFDM signal is 16.25 MHz.
 22. Thetransmitter of claim 21, wherein the transmit spectrum mask has aplateau of 18 MHz around a central frequency to enable a maximumamplitude for the modified WiFi OFDM signal and an attenuation of −99 dBover 6 MHz from edges of the plateau.
 23. The transmitter of claim 22,wherein the transmit spectrum mask includes sharp attenuations of −36 dBat the edges of the plateau over 500 kHz.
 24. The transmitter of claim21, wherein the modified WiFi OFDM signal has a symbol duration of 4 μs,including 3.2 μs as a useful signal duration and 0.8 μs as a guardinterval duration.
 25. The transmitter of claim 24, wherein the modifiedWiFi OFDM signal has a subcarrier spacing 312.5 kHz.
 26. The transmitterof claim 21, wherein the modified WiFi OFDM signal includes a data rateof 54 Mbps.
 27. The transmitter of claim 18, wherein an effectivespectrum occupied by the modified WiFi OFDM signal is 13 MHz.
 28. Thetransmitter of claim 27, wherein the transmit spectrum mask has aplateau of 18 MHz around a central frequency to enable a maximumamplitude for the modified WiFi OFDM signal and an attenuation of −99 dBover 6 MHz from edges of the plateau.
 29. The transmitter of claim 28,wherein the transmit spectrum mask includes sharp attenuations of −36 dBat the edges of the plateau over 500 kHz.
 30. The transmitter of claim27, wherein the modified WiFi OFDM signal has a symbol duration of 5 μs,including 4 μs as a useful signal duration and 1 μs as a guard intervalduration.
 31. The transmitter of claim 30, wherein the modified WiFiOFDM signal has a subcarrier spacing of 250 kHz.
 32. The transmitter ofclaim 27, wherein the modified WiFi OFDM signal includes a data rate of43.2 Mbps.
 33. The transmitter of claim 18, wherein the modified WiFiOFDM signal is modulated over 48 data carriers and uses 4 pilot carriersin the whitespace band.
 34. The transmitter of claim 18, wherein thewhitespace band extends over a band around a central frequency in aportion of the spectrum allocated to digital TV between 54 MHz and 698MHz.